1. Field of the Invention.
This invention relates generally to the mooring of oil tankers or other vessels in unprotected waters. More particularly the invention relates to a mooring system which combines a submerged buoyant mooring element anchored to the sea bed by catenary lines and a vessel provided with hoisting apparatus for raising the mooring element into engagement with a mooring recess in the bottom of the vessel hull.
2. Background Art.
In recent years, numbers of undersea oil and gas fields have been developed in offshore areas that are subject to extreme weather conditions. Oil or gas is typically delivered from a wellhead on the sea floor to a semi-permanently moored converted tanker or to a special purpose vessel known as a floating storage and off-loading (FSO) vessel or a floating production storage and off-loading (FPSO) vessel. These vessels are designed to remain on station permanently, unless oncoming severe storm or ice floe conditions threaten damage to or loss of the vessel. In such an event, the vessel is unmoored and sails or is towed away. Upon passing of the storm or ice floe condition, the vessel returns and is again moored above the wellhead.
A typical mooring system, such as for example described in U.S. Pats. No. 4,604,961 and No. 4,892,495, includes a buoyant mooring element that is connected to the wellhead by a flexible pipe and to the sea floor by a number of angularly spaced catenary lines. The system is arranged so that the weight of the portions of the catenary lines that are not resting on the sea bottom counteracts the buoyancy of the mooring element to maintain it normally at a predetermined submerged depth, called the stowed position, when no vessel is moored to it. This assures that the mooring element will not be a navigation hazard or be damaged by a collision.
Mooring is accomplished by pulling the submerged mooring element up to the vessel and securing it by mechanical means to a rotary turret mounted in a recess or well in the bottom of the vessel. This operation can take place only in a relatively low sea state. While the FSO or the FPSO vessel is away, the well is usually shut in; therefore the time required to wait for the sea state to subside after the storm has passed is additional lost production time from the field.
Another problem with the present technology is the possibility of impact damage to the vessel or the mooring element during mooring. A relatively long time is required to secure the mooring element to the turret after the hoist line has brought the mooring element close to the vessel bottom; during this time a number of rolling and pitching cycles of the vessel may take place in even a moderate sea state, and these movements can result in multiple collisions between the vessel and the mooring element.
Unmooring is accomplished by releasing structural connectors under load. This procedure has an inherent risk of structural deformation and damage to the mooring connectors during disconnect. Damage to the mechanical mooring components typically results in weeks of downtime, entailing costly oil field production shutdowns.